to "The
Advertisement Dept., The _Daily Wire_." Within were two plain sheets of
notepaper and a postal order. On one was written: "Dear Sir, please
insert the enclosed advts. in the personal column of your next
issue.--John Jones." On the other were two advertisements--

"R.F. You are closely watched. Don't forget 2315. Don't forget
2315. G.

"E. L27.14.5. To-morrow. B."

"Very curious," commented Foyle. "Copy them out carefully and have 'em
sent to the paper. They can't do any harm. Now let's get along."

The fog hung heavy over a muffled world as they walked down Victoria
Street. Green, whose wits were a trifle less supple than those of his
chief when imagination was required, put a question. Foyle answered
absently. The mysterious advertisements were not altogether mysterious
to him. He recalled the cipher that had been found at Grave Street, and
decided that there was at least room for hope in that direction.
Besides, there was at least one man now in custody who knew something of
the mystery, and, even if he kept his lips locked indefinitely, there
was a probable chance of a new line of inquiry opening when his identity
was discovered. And even if finger-prints and Pinkerton failed to
resolve that, there was still the resource of the newspapers. With a
photograph scattered far and wide, the odds were in favour of some one
recognising its subject.

As Foyle switched on the lights in the finger-print department, Green
sat down at a table and with the aid of a magnifying-glass carefully
scrutinised the prints which he carried on a sheet of paper. Ranged on
one side of the room were high filing cabinets divided into
pigeon-holes, numbered from 1 to 1024. In them were contained hundreds
of thousands of finger-prints of those known to be criminals. It was for
the detectives to find if among them were any identical with those of
their prisoner.

The whole science of finger-prints for police purposes resolves itself
into the problem of classification. It would be an impossible task to
compare myriads of records each time. The system employed was absurdly
simple to put into execution. In five minutes Green had the
finger-prints of the two hands classified into "loops" and "whorls" and
had made a rough note.

"W.L.W.L.L.
"L.W.W.L.W."

That done, the remainder was purely a question of arithmetic. Each whorl
was given an arbitrary number according to its position. A whorl
occurring in the f